Cambodia and Thailand agreed to an "immediate and unconditional ceasefire" from midnight (1700 GMT) on Monday, in a bid to halt their deadliest conflict in more than a decade after five days of fierce fighting that has displaced more than 300,

U.S.-backed talks to end the border war, in which militaries have killed dozens of people and displaced hundreds of thousands, began on Monday in Malaysia.

The Thai warning came as the U.S. expressed its grave concern about the most serious fighting in years between the Southeast Asian neighbors. The U.S. and China compete for influence in fast-growing Southeast Asia, and both have good relations with Thailand and Cambodia.

Thailand warned its conflict with neighboring Cambodia could “potentially develop into a war” as troops used rockets and artillery to shell targets along their contested border for a second

While the US and China urge a ceasefire, Thailand’s acting prime minister says he’s wary of Cambodia’s ‘acting in good faith’.

The leaders of Cambodia and Thailand agreed to a ceasefire on Monday effective midnight, in a bid to bring an end to their deadliest conflict in more than a decade after five days of fierce fighting.

"The U.S. already flunked the test and that should be a wakeup call," a former senior U.S. State Department official told Newsweek.

Cambodia and Thailand agreed to a ceasefire on Monday, set to start at midnight local time, after attending talks in Malaysia aimed at ending a 5-day border dispute that has left dozens dead.

People on both sides of the border share a cultural heritage much older than modern nation-states, dating back to the ancient Khmer civilization.